The present invention relates to television monitors having CRTs such as compact flat CRTs of small picture sizes, and more specifically to circuits for zooming correction in such CRT displays.
There are two different horizontal deflection systems, the conventional type and a separate type. A horizontal deflection circuit is separated from a high voltage generating circuit in the separate type, while the horizontal deflection circuit is not separated in the conventional type. The conventional type is adequate when a correction quantity for image error such as barrel distortion is small. When the correction quantity is great, however, adverse influence on the high voltage is significant, and accordingly the conventional type is not advisable.
The flat type CRT employs the separate type because the required correction quantity for distortion in the shape of an inverted trapezoid is very great. The separate type is advantageous in that it is free from interference between the high voltage supply circuit and the horizontal deflection circuit. Because of the separation, however, the horizontal deflection voltage remains constant while the high voltage varies, resulting in a phenomenon called zooming in which the picture size is varied.
A vertical deflection circuit is separate from the high voltage generating circuit of the CRT, and is also liable to cause zooming in the vertical direction. In the high voltage supply circuit, the anode voltage supplied to the anode varies in dependence on the anode current. In the vertical deflection circuit, on the other hand, the sawtooth current (vertical deflection current) of the vertical frequency period is supplied to the vertical deflection coil. The level of the vertical deflection current is constant irrespective of variation in the anode voltage. The result is vertical zooming in which the picture size varies in the vertical direction.
In such television receivers, therefore, the arrangement of the horizontal and vertical deflection circuits separate from the high voltage supply circuit tends to incur undesired variations in the horizontal and vertical picture sizes.
In general, the separate system employs a high voltage regulator to prevent this undesired zooming. This solution is useless, however, when the requirements for reducing power consumption and cost are critical as in the flat type CRTs. Another conventional solution is to increase the amount of overscan of the beam in each of the horizontal and vertical directions. However, this solution is still unsatisfactory in its high output and high cost, and incompetent for stabilization of the picture